Many knee replacement surgeries are performed annually. Typical knee replacements include a femoral component that is fixed to the patient's femur. Knee replacements also include a tibial component that is fixed to the patient's tibia. A bearing insert is sandwiched between the femoral component and the tibial component and acts a replacement for the patient's cartilage. Most knee replacements are glued to the patient's bones with bone cement. Some knee replacements are simply press fit in place without cement. Either way, most knee replacements last 10-15 years, while some fail earlier for various reasons.
When a knee replacement fails, it must be fully revised. Over the life of a knee replacement, the patient's bone tissue can deteriorate resulting in bone loss. As such, when knee replacements are revised, they typically need additional mechanical fixation to compensate for the bone loss. This additional mechanical fixation takes the form of a stem that extends from the tibial component (or the femoral component) into the patient's bone canal.
Conventional stems are cylindrical. However, bone canals are rarely cylindrical. In addition, the portions of the bone canals that engage the cylindrical stems are typically tapered. Such a configuration allows for voids between the walls of the bone canal and the cylindrical stem, which allow the cylindrical stem to move or shift within the bone canal. Such movement or shifting can impede the cylindrical stem's ability to provide sufficient mechanical fixation, which in turn, can compromise the life of the revision knee replacement and can even cause pain to the patient.
Several solutions have been presented to compensate for the suboptimal fit of the cylindrical stem within the patient's bone canal. One solution uses bone cement to fill the voids between the cylindrical stem and the wall of the bone canal. However, all knee replacements (including revision knee replacement) have a limited life expectancy. In the event of knee replacement failure, the bone cement makes it more difficult to remove the previous knee replacement and contributes to even more bone loss with subsequent decreased structural support from the bone.
Another solution uses a hollow metallic support cone. The revision stem extends through the metallic support cone but is not affixed to the metallic support cone. Also, the metallic support cone is not fixed to the tibial component. This configuration allows for movement of the cylindrical stem relative to the metallic support cone, which in turn allows for movement of the revision stem relative to the bone canal. Cement is often used to fill this void. In addition, due to the unique nature of revision knee surgery, this system requires a large inventory. For example, upwards of 50 stems and 10 cones are required to ensure that the correct components are matched to the patient's specific geometry and fixation needs.
Another solution extends the length of the cylindrical stem so that it projects further into the patient's bone canal. Although this configuration may provide more support for the knee replacement, the extra length projecting deeper into the patient's bone canal can cause pain.
Yet another solution uses a stepped metallic sleeve that is fixed to the cylindrical stem. This configuration is not able to “wedge” into the patient's bone canal due to its “stepped” structure, thereby limiting its ability to support the prosthesis. In addition, the metallic sleeve is not versatile and is compatible with only one knee replacement system. Furthermore, due to the unique nature of revision knee surgery, this system requires a large inventory. For example, upwards of 50 stems and 10 sleeves are required to ensure that the correct components are matched to the patient's specific geometry and fixation needs.